Lesson 9: Properties of Water

Transcription

1 Lesson 9: Properties of Water Slide 1: Introduction Slide 2: Oil and water don t mix Fascinating Education Script Fascinating Chemistry Lessons As we ve seen from trying to heat water, pulling water molecules apart requires a great deal of heat energy. It s understandable, then, why hydrogen bonds are too strong for non-polar molecules like methane, ethane, propane, butane, pentane, and hexane to break apart and dissolve in water. Even longer chain hydrocarbons cannot break the hydrogen bonds between water molecules. Consequently, when mixed together, oils and water don t mix. Here for example, is freshly made chicken soup. Up top is the oil and fat consisting of long chain hydrocarbons, and underneath is the watery part of the soup. Since the oils and fats are less dense than water, they float on top of the water. Olive oil and vinegar demonstrate the same thing, with the less dense olive oil floating atop the vinegar, which is acetic acid dissolved in water. Slide 3: Micelles and soaps The only way to get non-polar molecules in between water molecules instead of floating on top of them is to use soap. Soap molecules consist of long chains of non-polar hydrocarbons capped on one end by a polar carboxyl group where the end carbon atom is holding on to two oxygen atoms. One of the oxygen atom is itself bonded ionically to a sodium or potassium atom.

2 When soap is mixed with water, water molecules point their negative sides toward the sodium atom and pull it away from the oxygen atom. The sodium atom leaves its single outer electron with the oxygen atom, becoming a positive cation in the process, and leaving the carboxyl end of the soap molecule an electrically negative anion. The negative carboxyl end of the soap molecule is then attracted to the positive side of water molecules which keeps the soap molecule suspended in among the water molecules. Meanwhile, the long hydrocarbon chains, being excluded from water, cluster together to form microscopic lakes, called micelles, that remain suspended in the water. The micelles trap bits of oily dirt while the polar ends of the soap molecules keep the micelles suspended in water. When water drains down the sink, it carries the micelles of grease with it. Slide 4: Viscosity Even though a single hydrogen bond between two water molecules is stronger than a single London dispersion force, when there are enough London dispersion forces, non-polar molecules can stick together tighter than water molecules. One way to measure the strength of intermolecular bonding is viscosity. Viscosity is a measure of resistance to flow. Drop two paper clips simultaneously into a glass of water and a glass of cooking oil and see which one sinks faster. In order for the paper clips to flow through either liquid, they have to break each liquid s intermolecular bonds. Cooking oil consists of long chains of hydrocarbons sticking to each other with London dispersion forces. As this picture shows, it takes a paper clip longer to sink through cooking oil than through water, indicating that the viscosity of cooking oil is greater than the viscosity of water.

3 The long chains of hydrocarbons in cooking oil allow for so many London dispersion forces that, together, they re stronger than water s hydrogen bonds. How would you reduce the viscosity of water? Sure, heat the water. Heat will break many of the intermolecular bonds so the paper clip won t have to expend as much energy breaking them and will thus sink faster. There is a lab that goes with this slide. You can access it on the Labs Page of fascinatingeducation.com. Slide 5: Surface tension And yet a paper clip will float on the surface of water, but not on the surface of oil. Even though this paper clip is denser than water, it floats on the surface of the water because of surface tension created by hydrogen bonds. Surface tension is why, on most waterproof surfaces, like a leaf, water forms little beads. A drop of oil won t bead up; it ll form a thin coating instead. The reason water beads up is that water molecules below the surface are drawn in all directions to other water molecules. But at the surface, water molecules can only be attracted to molecules beneath the surface. Therefore, no matter where water molecules are along the surface, they re drawn inward, forcing the water molecules into a round water droplet. In a sense, surface tension gives the surface of water a thin skin, allowing water to bulge upward over the rim of a glass, or just hang there without dripping. Surface tension allows things like paper clips to rest on the surface of water.

4 Gerris bugs take advantage of surface tension to push against the surface and dart across water at upwards of 400 miles per hour. The forces creating surface tension are acting horizontally between water molecules at the surface to prevent things like paper clips from poking through the surface of the water, but the forces are also acting vertically, pulling the surface of the water downward. For example, inside a hollow tube, water molecules at the surface use their hydrogen bonds to attach to the walls of the glass tube. The rest of the water molecules at the water s surface hang on to those water molecules with their horizontally-directed hydrogen bonds, but all the water molecules below the surface of the water are also exerting a downward pull and causing the column of water to sag in the middle as a meniscus. The downward forces are what prevent you from lifting a thin layer of surface water molecules out of the water with a plastic loop. What could we add to the water to break up the surface tension and allow the paper clip to sink and you to lift a thin layer of water molecules out of the water? Slide 6: Soap Soap, but how does soap do that? After the sodium ions are pulled off the polar carboxyl ends of the soap molecules, the now negative ends of the long chain hydrocarbons spread out among all the water molecules. Those that make their way to the surface of the water are able to slip between the water molecules. Their long, nonpolar chains, being excluded from the water, poke up out of the water. By separating the water molecules at the surface, soap molecules lower water s surface tension and allow the paper clip to sink.

5 You can also see the effects of lowering surface tension by sprinkling pepper onto a bowl of water, and then touching the center of the pepper with a drop of dish washing liquid. The sudden lowering of surface tension in the center allows the water molecules around the center to yank the pepper toward the periphery. In addition, soap breaks all those hydrogen bonds holding down the surface layer of water molecules. Now the plastic loop can lift the very top layer of water molecules from the surface of the water. And, by trapping the very thin layer of water molecules between two layers of soap molecules, the water molecules won t evaporate and pop. Notice that each soap layer is oriented so that its polar side faces the polar water molecules lying between the two soap layers. By preventing the water from evaporating, the soapy film now has time, when blown from the plastic loop, to seal itself into a minimum volume, namely, a spherical bubble. Slide 7: Density of water vs. ice We know from lesson 7 that water expands when it freezes. So which is denser, a drop of water or the same drop of water when frozen? Density is mass per volume, with mass in the numerator and volume in the denominator. A typical expression of density might be 50 milligrams per cubic centimeter.

6 If water expands when it freezes, then the same mass of water in the numerator must be occupying a larger volume when it freezes. With a larger denominator, the density of Ice must be less than water. If ice is less dense than water, what do you predict about ice in water? Ice should float on water, which it does. Here, for example, is an iceberg floating in the ocean. What do you notice about this iceberg? Most of the iceberg is below the surface of the water. Why? If ice is less dense than water, why doesn t it all float above the water? And why doesn t this steel ship sink? Steel is denser than water. How could this aircraft carrier weighing 120,000,000 pounds not sink? What determines whether something will float, and if it does float, what determines how high it floats in the water? All of these questions are easily answered once you know how much water is being pushed aside by the object, and how much that water weighs. There is a lab that goes with this slide. You can access it on the Labs Page of fascinatingeducation.com. Slide 8: Displacing water When a boat floats in the water, it pushes aside, or displaces, a certain amount of water. The column of water beneath the volume of water being displaced only, shall we say, agreed to support the weight of displaced water but no more. Whether it s water or the hull of a ship is unimportant, so long as its weight is not more than what an equal volume of water would weigh. Otherwise, the ship will sink. The upward force exerted by the column of water below the displaced water is called buoyancy force. When doing the calculations, the amount of water displaced is that volume of the boat below the surface of the water, but what is the mass of the boat? Is it the mass of the entire boat, above and below the surface, or just the mass below the surface? The whole boat. So if the

7 mass of the entire boat is greater than the mass of the water displaced by the boat, the boat will sink until the mass of the boat equals the mass of the water displaced by the boat, at which point the boat will stop sinking and float at that level. How is this seaplane able to float on water? Same thing: buoyancy. The weight of the plane is less than the weight of the water displaced by the two floats. Does the Gerris bug float because of buoyancy or surface tension? It moves across the surface of the water by pushing off against surface tension, but it stands on the water because of buoyancy. The undersurface of Gerris legs are made up of millions of tiny hairs that trap air bubbles which act just like the floats on a seaplane. Slide 9: Floating an iceberg To figure out how high this 100 ton iceberg floats, imagine lifting the iceberg out of the water and lowering it back into the water. At first, it displaces only, say, 45 tons of water, but as the iceberg is lowered further, the amount of water pushed aside increases, until finally, the weight of the water displaced equals the weight of the iceberg, namely, 100 tons, and the iceberg stops sinking. Slide 10: Salt water vs. fresh water Salt water is denser than fresh water. Which means, for the same volume, salt water weighs more than fresh water. So why do ships ride higher in the salt water of Puget Sound than the fresh water of Lake Michigan? Salt water is denser than fresh water because the added salt makes every cubic foot of salt water heavier. As the boat is lowered into Puget Sound, the weight of the displaced salt water quickly equals the weight of the boat. In Lake Michigan, however, the boat has to be lowered further into the water before the weight of the lighter, fresh water equals the weight of the boat.

8 People easily float in the Dead Sea for the same reason. The Dead Sea is extremely salty, and therefore very dense. As your body is slowly lowered into the Dead Sea, it quickly displaces enough water to equal your body weight, and you float effortlessly. There is a lab that goes with this slide. You can access it on the Labs Page of fascinatingeducation.com. Slide 11: Sinking and raising ships Why do ships sink? Ships sink because they suddenly become heavier than the water they displace. This can happen if the ship becomes too heavy, or the water becomes too light. A ship might become too heavy if it springs a leak. Suddenly, the combined weight of the ship and the incoming water is now heavier than the weight of the water displaced by the ship. So the ship sinks. Here is a ship sailing over a column of hot bubbling water rising up from a volcanic eruption on the ocean floor. Why would this boat suddenly sink? The boat would sink for the same reason any boat sinks: the boat becomes heavier than the water it displaces. However, in this case, the boat didn t become heavier; the water became lighter from all the gas bubbles and heat erupting from the volcano. That s why this orange ball sinks when the water is heated. Before heating, the ball on the left has exactly the same density as the water at that particular level. It weighs exactly the same as the water it displaces at that level. As the water is heated, the density of the water falls. The ball is now heavier than the water it displaces at that level, so it sinks to denser water until the weight of the water displaced equals the weight of the ball.

9 Slide 12: Fish survival in a frozen lake Here is a graph of water temperature versus density. As water temperature increases, density falls. But now let s go backwards. Begin over toward the right with hot water beyond 40 degrees Celsius, 104 degrees Fahrenheit. As the temperature cools and the water molecules slow down, water becomes denser. The peak density occurs at 4 degrees Celsius. As the temperature falls below 4 degrees Celsius, however, some of the water molecules slow down enough that, for a split second, they begin to form crystals of ice. Ice is less dense than water, so between 4 degrees Celsius and 0 degrees, where water freezes, the density of water drops as water molecules spend more and more time in a crystalline shape. Finally, when ice does crystalize, the density plummets well below the density of water at any temperature. So ice always floats in water no matter what the temperature. That s why, when this lake freezes over, the surface of the lake freezes first. Yes, the surface of a lake is the first to experience the cold wintry wind, but the more important reason for the surface freezing first is that as water begins freezing, it becomes less dense and floats to the surface. How are fish able to survive in a frozen-over lake? When water temperature is actually measured in a lake that s frozen over, the water temperature right below the ice is 32 degrees Fahrenheit, 0 degrees Celsius, but then rises the deeper you go. If the lake is deep enough, the water temperature at the bottom of the lake can get up to a relatively comfortable for a fish degrees Fahrenheit, 4 degrees Celsius. Why does water temperature rise the deeper you go? Because water is densest at 4 degrees Celsius, 39 degrees Fahrenheit, and dense water sinks. In addition, the layer of ice, and the layer of air right underneath the ice, serve to insulate the water below from the cold wintry air above. Slide 13: Determining the density of the ocean

10 The density of ocean water depends on more than temperature alone; it also depends on its salt concentration. Suppose you tossed a stone overboard in the ocean. How far would it descend? If water beneath the surface were the same density all the way down to the bottom of the ocean, then once the stone started descending, it would continue to sink to the bottom of the ocean because its weight would always be greater than the weight of the water it displaced. However, if the density of the water beneath the surface increased the deeper you got, then the stone would become suspended when its density equaled the density of the surrounding water. It turns out that that for the first kilometer, which is a little over half a mile, the density of ocean water continues to rise and then, surprisingly, remains pretty constant down to the ocean floor. You d think the weight of all the ocean water would compress the deep-lying water, but water, like all liquids, is non-compressible because the molecules in a liquid are already close together and cannot be pushed closer together. This is good news for fish because if water were compressible, water in the deep ocean would be too dense to swim through, or in scientific terms, too viscous. So why does it hurt to do a belly flop? Two reasons. One reason is that striking the surface of the water with the whole surface area of your body spreads out the force. Unlike putting your palms together and slicing through the surface of the water in a dive, slapping the water surface with your body doesn t concentrate the force enough to split the hydrogen bonds at the water s surface. The other reason is that water is non-compressible and unless you do break the surface of the water and slide between water molecules, striking a block of water is like striking concrete. For the same two reasons, road barriers made of plastic filled with water are able to withstand large trucks crashing into them.

11 Slide 14: What you know so far 1. Hydrogen bonding is so strong that non-polar molecules cannot break them and mix in among water molecules. As a result, oil and water don t mix. 2. Soap molecules are able to mix in among water molecules because of soap s polar heads. The hydrocarbon tails, however, remain excluded from water molecules and, being non-polar, group together as round pockets called micelles. 3. Dirt is generally non-polar and hence, collects inside micelles. The micelles remain suspended in water because the polar carboxyl groups on each soap molecule are attracted to the polar water molecules. 4. Viscosity is resistance to flow resulting from the intermolecular attraction between molecules. Viscosity is higher for long-chain non-polar molecules, like long-chain hydrocarbons, than for water, because hydrocarbon s London dispersion forces are, in total, greater than water s hydrogen bonds. 5. Water s surface tension is due to the strong inward attraction of its surface molecules -- inward because there are no water molecules above the surface to be attracted to. Slide 15: What you know so far 6. Surface tension causes water to bead up and allows a paper clip to float on water. Gerris bugs float on trapped bits of air, allowing water s buoyancy, not surface tension, to keep it afloat. 7. Vertically directed hydrogen bonding pulls surface molecules downward and prevents them from being lifted off the surface. Soap bubbles break the vertically-directed hydrogen bonding and allows a thin layer of surface water molecules to be trapped between two layers of soap molecules so that a thin layer of water molecules can be lifted off water s surface. By being trapped between soap bubbles, water molecules don t evaporate and burst all the soapy film. 8. The density of ice is lower than the density of water. 9. An object floats if the water the object displaces weighs the same as the object. Slide 16: What you know so far 10. Dissolved salt makes water denser, so ships float higher in salt water than fresh water because the ships don t need to displace as much water to equal the weight of the ship. 11. Hot water is less dense than cold water, so heating water forces a floating object to float deeper in the water in order to displace more water. If the density of the water drops enough,

12 the floating object may have to displace a volume of water equal to the volume of the object, forcing the floating object to sink below the surface. 12. Water is densest at 4 o Celsius, 39 o Fahrenheit. Any colder, and some of the water molecules begin forming less dense ice crystals that float to the water s surface. So water between 0 o and 4 o Celsius is heavier than ice and sinks so that the densest water at 4 o Celsius lies at the bottom of the lake. 13. Because water molecules are already very close together, from about 1 kilometer, or about half a mile below the ocean s surface to the bottom of the ocean, the weight of ocean water cannot compress water molecules any further, so the density of ocean water remains the same from about a kilometer or half a mile downward. NOTE: There are math problems associated with this lesson. You will find these problems in the lessons and on the math problems page.

13 Chemistry Lesson 9 Lab Note: Access the full lab instructions and explanations by clicking the Chemistry Labs tab on the Members menu of the site or visit you will need a special password for the labs. Please ask an adult to provide this for you.

14 Chemistry Lesson 9 Lab Note: Access the full lab instructions and explanations by clicking the Chemistry Labs tab on the Members menu of the site or visit you will need a special password for the labs. Please ask an adult to provide this for you.

15 Chemistry Lesson 9 Lab Note: Access the full lab instructions and explanations by clicking the Chemistry Labs tab on the Members menu of the site or visit you will need a special password for the labs. Please ask an adult to provide this for you.